Apparatus for providing timing jitter tolerant optical modulation of a first signal by a second signal

ABSTRACT

Apparatus for providing timing jitter tolerant optical modulation of a first signal by a second signal, the first signal having a first wavelength, the second signal including a plurality of second signal pulses having a second pulse shape and a second wavelength. The apparatus includes a first signal input port, a second signal input port, a coupler, a grating and a non-linear optical device. The apparatus is configured to direct the second signal at the second signal input port to the non-linear optical device via the coupler and the grating, and to direct the first signal at the first signal input port to the non-linear optical device. The grating is a superstructured fibre Bragg grating that converts the second signal pulses into intermediary pulses each having an intermediary pulse shape. The intermediary pulse shape is such that it provides a switching window within the non-linear optical device.

CROSS REFERENCE TO RELATED APPLICATIONS

The application being filed herewith is a U.S. National Stage filingunder 35 U.S.C. § 371 (et seq.) of Patent Cooperation Treaty (“PCT”)Patent Application serial number PCT/GB02/01199, filed 15 Mar., 2002,which in turn claims priority to United Kingdom (Great Britain) PatentApplication Serial Number GB 0106553.1, filed 16 Mar. 2001.

FIELD OF THE INVENTION

This invention relates to optical modulators, which incorporatepulse-shaping fibre Bragg gratings, and which are tolerant to timingjitter.

BACKGROUND OF THE INVENTION

In order to ‘synthesize’ a particular optical pulse form one needs to beable to reliably define the amplitude and phase profile of an opticalfield. The general approach is to generate pulses with a well-definedpulse form and to then pass the pulse through some pulse shaping elementwith an appropriately designed transfer function to re-phase andre-shape the incident spectrum so as to obtain the desired outputoptical field. The pulse-shaping element can have a pure linear responsesuch as a filter with a suitably complex response, or might additionallyinclude a non-linear element, e.g. an optical fibre or an aperiodicquasi-phase matched structure, to allow the controlled generation offrequency components outside the frequency spectrum of the inputpulse-form.

The most commonly used technique is a simple linear filtering techniquein which the frequency components of a short pulse are spatiallydispersed using bulk gratings, and then filtered by means of amplitudeand phase-masks positioned within a Fourier optical 4 f set-up.Microlithographically fabricated spatial-masks, segmented liquid crystalmodulators, or acousto-optic modulators have been used as spatialfilters, the latter two approaches allowing for programmability anddynamic reconfigurability of the pulse shaping response. Whilstimpressive results are possible with this approach, the hardware itselfis somewhat bulky, lossy and expensive and does not lend itself to readyintegration with waveguide devices. These issues have prompted thesearch for other technical approaches to the problem such as the use ofarrayed waveguide gratings, and arrays of fibre delay lines.

Single channel data rates approaching the Tbit/s level have now beenreported for optical time division multiplexing (OTDM) systems. Thesesingle channel data rates place increased demands and tolerances on thetechniques used to multiplex and demultiplex the optical data bits.Consider for example the case of optical demultiplexing. As OTDM datarates increase, and the pulses used get correspondingly shorter, thesynchronization requirements placed on the locally generated pulses usedto control the switch operation can become a limiting practical issue.The key to reducing time jitter tolerance in such devices is toestablish a rectangular temporal switching window. This reduces theabsolute accuracy for temporal bit alignment and provides optimalresilience to timing jitter-induced errors. Fibre based non-linearoptical loop mirror (NOLM) demultiplexing schemes that provide bothgood, ultra-fast performance and tolerance to timing-jitter of either orboth of the control and data signals have been demonstrated previously.These schemes use the difference in group velocity and the resultant‘walk-off’ between the control and data signals within the non-linearoptical device to define the rectangular switching window. Thisconsequently requires tight specification and control of both the dataand signal wavelengths, and the dispersion characteristics of the fibre.Whilst this approach is applicable to fibre based switches, it iscomplex and cannot be applied to switches based on highly non-linearsemiconductors and within which there are no appreciable dispersivepropagation effects over the length scales of relevance. Simple, robusttechniques that can help reduce time jitter tolerances and that areapplicable to a variety of switching mechanisms are thus of greatinterest.

It is an aim of the present invention to obviate or reduce the abovementioned problems.

SUMMARY OF THE INVENTION

According to one non-limiting embodiment of the present invention, thereis provided apparatus for providing timing jitter tolerant opticalmodulation of a first signal by a second signal, the first signal havinga first wavelength, the second signal comprising a plurality of secondsignal pulses having a second pulse shape and a second wavelength, andthe apparatus comprising a first signal input port, a second signalinput port, a coupler, a grating and a non-linear optical device, theapparatus being configured to direct the second signal at the secondsignal input port to the non-linear optical device via the coupler andthe grating, and to direct the first signal at the first signal inputport to the non-linear optical device; the grating being asuperstructured fibre Bragg grating that converts the second signalpulses into intermediary pulses each having an intermediary pulse shape;and the intermediary pulse shape being such that it provides a switchingwindow within the non-linear optical device.

The first signal can comprise a plurality of first signal pulses. Thefirst signal can be a continuous wave signal such as an un-modulatedlaser beam. The switching window can be rectangular, Gaussian, or anyother user-defined shape.

In one embodiment of the invention, the first signal comprises aplurality of first signal pulses, the grating is defined by a gratingimpulse response, the intermediary pulse shape is defined by theconvolution of the second pulse shape and the grating impulse response,and the switching window is a substantially rectangular switching windowwhich provides tolerance to a variation in arrival time of the firstsignal pulse at the first input port and the second signal pulse at thesecond input port substantially equal to the width of the rectangularswitching window.

In another embodiment of the invention, the first signal comprises aplurality of first signal pulses each having a width, the grating isdefined by a grating impulse response, the intermediary pulse shape isdefined by the convolution of the second pulse shape and the gratingimpulse response, the grating being such that the intermediary pulseshape is a substantially rectangular pulse, and in which the apparatushas a tolerance to a variation in arrival time of the first pulse at thefirst input port and the pulse at the second input port substantiallyequal to the width of the substantially rectangular pulse minus thewidth of the first signal pulse.

The apparatus of the invention may be one in which the coupler is acirculator. The coupler may be an optical fibre coupler.

The apparatus may comprise an optical switch, the optical switch beingsuch that it comprises the non-linear optical device.

The non-linear optical device may be a holey fibre. The holey fibre maycomprise glass. The glass may be silica, a silicate glass, or a compoundglass. Alternatively, the holey fibre may comprise a polymer.

The holey fibre may comprise a core and a cladding, in which thecladding comprises a plurality of holes arranged around the core, and inwhich the core has a diameter less than 10 um. The core may have adiameter less than 5 um. The core may have a diameter less than 2 um.

The holey fibre may comprise a dopant, the dopant being selected fromthe group comprising Ytterbium, Erbium, Neodymium, Praseodymium,Thulium, Samarium, Holmium Dysprosium, Tin, Germanium, Phosphorous,Aluminium, Boron, Antimony, Uranium, Gold, Silver, Bismuth, Lead, atransition metal, and a semiconductor. The above list of elementsincludes associated chemical compounds of the element, and in particularall-oxide forms.

The non-linear optical device may be, or may comprise, a semiconductoroptical amplifier.

The non-linear optical device may comprise a lithium niobate channelwaveguide, or a lithium niobate planar waveguide.

The non-linear optical device may comprise a periodically poled lithiumniobate channel waveguide or a periodically poled lithium niobate planarwaveguide.

The non-linear optical device may be an optical switch, a holey fibre, apoled-fibre, a potassium titanyl phosphate (KTP) or other crystallinewaveguide, a periodically poled KTP or other crystalline waveguide, anon-linear optical loop mirror, a Kerr gate, an optical fibre, anon-linear amplifying loop mirror, or a non-linear optical modulator.

The optical non-linearity used within the non-linear optical device maybe based on second-order (χ(2)), or third-order (χ(3)) non-lineareffects. The specific manifestation/use of the non-linearity might be interms of Self-Phase Modulation (SPM), Cross-Phase Modulation (CPM),Four-Wave Mixing (FWM), parametric frequency conversion, second harmonicgeneration, third harmonic generation, sum frequency generation,difference frequency generation, supercontinuum generation, cascadedsecond order effects, or some combination thereof. Other opticalnon-linearites that might be used include Raman and Brillouin effects,cross gain modulation and two photon absorption.

The apparatus may be configured to modulate the first signal.

The apparatus may be configured to demultiplex the first signal.

The apparatus may comprise an actively mode-locked fibre laser.

The apparatus may comprise an interferometer comprising a first arm anda second arm, and in which the first arm comprises the non-linearoptical device.

The apparatus may comprise a filter, and in which the filter is awavelength selective filter.

The apparatus may comprise a polarizing element, and in which thepolarizing element is a polarizer or a polarization beam splitter.

The apparatus may comprise a clock generator. The clock generator may bea short-pulse generator selected from the group comprising a mode-lockedfibre laser, an actively mode-locked fibre laser, a generator comprisingan electro-absorption modulator and a laser, a generator comprising anelectro-optic modulator and a laser, and a gain-switched laser diode.

The clock generator may comprise a means for pulse compression such asdispersion compensator fibre, a chirped fibre Bragg grating, adispersion decreasing fibre, an optical amplifier, a Raman amplifier, anoptical switch, an optical pulse compressor, or some combination ofthese devices.

The apparatus may comprise a plurality of non-linear optical devices andbe configured to direct the second signal at the second signal inputport to each of the non-linear optical devices. The apparatus may beconfigured as an optical multiplexer. The apparatus may be configured asan optical demultiplexer. The apparatus may be configured as an inversemultiplexer. The apparatus may comprise a switch and a control input forcontrolling the switch.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described solely by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 shows apparatus according to the present invention for providingjitter tolerant optical modulation of a first signal by a second signal;

FIG. 2 shows an embodiment of the present invention in which theintermediary pulse shape is defined by the convolution of the secondpulse shape and the grating impulse response;

FIG. 3 shows a grating impulse response;

FIG. 4 shows an intermediary pulse shape;

FIG. 5 shows a substantially square switching window;

FIG. 6 shows a first signal pulse arriving prior to a second signalpulse;

FIG. 7 shows a first signal pulse arriving after a second signal pulse;

FIG. 8 shows the impulse response of an optical switch;

FIG. 9 shows an embodiment of the present invention configured toprovide a substantially rectangular pulse;

FIG. 10 shows a grating impulse response;

FIG. 11 shows an embodiment comprising an optical switch;

FIG. 12 shows the end face of a holey fibre;

FIG. 13 shows apparatus according to the present invention andcomprising a holey fibre;

FIG. 14 shows apparatus according to the present invention andcomprising an interferometer;

FIG. 15 shows apparatus according to the present invention configured asan optical demultiplexer;

FIG. 16 shows apparatus according to the present invention andconfigured as an optical multiplexer;

FIG. 17 shows apparatus according to the present invention andconfigured as an inverse multiplexer;

FIG. 18 shows apparatus according to the present invention and furthercomprising a switch;

FIG. 19 shows a multiplexer according to the present invention withregenerative properties;

FIG. 20 shows a DWDM multiplexer according to the present invention;

FIG. 21 shows apparatus according to the present invention utilizingcontinuous wave switching;

FIGS. 22 to 30 show the results of calculation on pulse shaping filtersaccording to the present invention;

FIG. 31 shows apparatus according to the present invention comprising amode locked ring laser;

FIGS. 32 to 39 show experimental and theoretical results on theperformance of a rectangular pulse shaping filter according to thepresent invention;

FIG. 40 shows a test set up for characterizing apparatus according tothe present invention;

FIGS. 41 to 44 show theoretical and experimental performance ofapparatus according to the present invention;

FIG. 45 shows a test set up for characterizing apparatus comprising asemiconductor optical amplifier according to the present invention;

FIGS. 46 and 47 shows experimental results for the semiconductor opticalamplifier;

FIG. 48 shows a definition of a switching window; and

FIG. 49 shows a typical transmission response of a fibre-basednon-linear optical device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows apparatus for providing timing jitter tolerant opticalmodulation of a first signal 1 by a second signal 2. The first signal 1has a first wavelength 3. The second signal 2 comprises a plurality ofsecond signal pulses 4 having a second pulse shape 5 and a secondwavelength 6. The apparatus further comprises a first signal input port8, a second signal input port 9, a coupler 10, a grating 11 and anon-linear optical device 12. The apparatus is configured to direct thesecond signal 2 at the second signal input port 9 to the non-linearoptical device 12 via the coupler 10 and the grating 11, and to directthe first signal 1 at the first signal input port 8 to the non-linearoptical device 12. The grating 11 is a superstructured fibre Bragggrating that converts the second signal pulses 4 into intermediarypulses 13 each having an intermediary pulse shape 14. The intermediarypulse shape 14 is such that it provides a switching window 19 within thenon-linear optical device 12.

The switching window 19 may be a substantially rectangular switchingwindow 50 shown with reference to FIG. 5. The switching window 19 may bealternatively be Gaussian, or any other user-defined shape.

The apparatus can be configured such that the apparatus has a singleoptical input port so that first and second signals 1 and 2 enter theapparatus through the same input port.

The apparatus may further comprise a filter 21 as shown in FIG. 1.

The coupler 10 can be a circulator, an optical fibre coupler or a beamsplitter.

The first signal 1 is shown as comprising a plurality of first signalpulses 15. These first signal pulses 15 interact with the intermediarypulses 13 within the non-linear optical device 12, providing an outputsignal 18 comprising a plurality of output pulses 16 at the output port17. The first signal 1 is shown as having pulses in time slots t1, t2,t3 and t5; the second signal 2 is shown as having pulses in time slotst1, t3 and t5; and the output signal 18 is shown as having pulses intime slots t1, t3 and t5—ie corresponding to the overlap in time of thefirst signal pulses 15 and the intermediary pulses 13 within thenon-linear optical device 12. The substantially rectangular switchingwindow 50 provided by the intermediary pulse shape 14 helps to reducedistortion of the first signal pulses 15 within the non-linear opticaldevice 12 and provides tolerance to jitter in the arrival times of thefirst signal pulses 15.

Although FIG. 1 shows the first signal 1 as comprising a plurality offirst signal pulses 15, the first signal 1 can alternatively be acontinuous wave signal such as an un-modulated laser beam.

The first wavelength 3 can be different from the second wavelength 6. Inthis case, the non-linear optical device 12 is preferably asemiconductor active element or an optical fibre such as the holey fibre120, and the filter 21 is preferably a wavelength selective filter suchas a fibre Bragg grating or a thin-film filter.

The first wavelength 3 can be the same as the second wavelength 6. Inthis case, the non-linear optical device 12 is preferably such that itsoutput state of polarization changes in response to the intermediarypulses 13, and the output filter 21 is preferably a polarizer or apolarizing beam splitter.

The first signal 1 can be a data signal and the second signal 2 can be aclock signal. In this case the apparatus provides a demultiplexingfunction. Alternatively, the first signal 1 can be a clock signal andthe second signal 2 can be a data signal whereupon the apparatusprovides a multiplexing function.

FIG. 2 shows one embodiment of the invention in which the first signal 1comprises a plurality of first signal pulses 15. The grating 11 isdefined by a grating impulse response 30 as shown in FIG. 3. As shown inFIG. 4, the intermediary pulse shape 14 is defined by the convolution ofthe second pulse shape 5 and the grating impulse response 30. Thegrating impulse response 30 is selected to provide the rectangularswitching window 50 shown in FIG. 5 in the non-linear device 12, whichin FIG. 2 is shown as comprising a semiconductor optical amplifier 20.

The apparatus may further comprise the filter 21.

The semiconductor optical amplifier 20 can be part of a terahertzoptical assymetric demultiplexer (TOAD), semiconductor laser amplifierin a loop mirror (SLALOM), an ultra-fast non-linear interferometer(UNI), a gain transparent ultra-fast non-linear interferometer (GT-UNI).

The rectangular switching window 50 provides tolerance to a variation inarrival time of the first signal pulse 15 at the first input port 8 andthe second signal pulse 4 at the second input port 9 substantially equalto the width 51 of the rectangular switching window. 50 minus the widthof the first signal pulse 15.

FIG. 6 shows an example in which the first signal pulse 15 arrives at atime 61, the second signal pulse 4 arrives at a time 62, and in whichthe rectangular switching window is the width 51. FIG. 7 shows anotherexample in which the first signal pulse 15 arrives at a time 71, thesecond signal pulse 4 arrives at a time 72, and in which the rectangularswitching window is the width 51. FIGS. 6 and 7 illustrate the maximumpossible variation in the arrival times 61 and 62 such that thenon-linear optical device 12 does not distort the first signal pulse 15by virtue of timing jitter.

The term switching window can be defined in a number of ways. A firstdefinition shown in FIG. 21 is that the ‘continuous wave switchingwindow’ 210 is defined by the pulse shape 210 when driven by theintermediary pulse 13 in response to the second signal pulses 4, and inwhich the first signal 1 at wavelength 3 is a continuous wave beam. Whenusing the apparatus with a first signal 1 comprising first signal pulses15, the switching window is given by the width of the ‘continuous waveswitching window’ 210 minus the width of the first signal pulse 15. Asecond definition can be couched more directly in terms of the systemperformance itself as illustrated in FIG. 48 which shows a plot of BitError Rate (BER) 480 versus relative time delay 481 of the pulsesassociated with the first and second signal pulses 15, 4. Bit error ratemeasurements provide a measure of the quality of the pulse switchingprovided by the apparatus, the higher the bit error rate the worse theswitch performance. An error rate of 1 in 10⁻⁹ is often referred to aserror-free and taken to represent the lower bound on acceptableperformance of telecommunication apparatus. Thus an alternative systemdescription of timing window for the switch is the total length ofrelative timing delay 482 over which error free performance can beachieved. The BER increases rapidly at the rising and trailing edges ofthe timing window. In practice the two definitions of timing values giveroughly equivalent values under the two definitions, particularly forrectangular switching windows and we use both definitions herein. Morespecifically we tend to use the first definition when the intermediarypulse form is itself substantially rectangular in shape, and the laterdefinition for the more general case.

The embodiment shown in FIG. 2 is representative of switching approachesin which the required intermediary pulse form required to provide asubstantially rectangular switching window is not a rectangular pulse,but some other pulse form that compensates for non-instantaneousnon-linear responses (such as cross gain modulation) and dispersiveeffects within the non-linear optical device 20.

In order to establish the intermediary pulse shape 14 it is necessaryfirst to characterise the impulse response 80 shown in FIG. 8 of thenon-linear optical device 12 as a function of incident pulseenergy/power, and then to use this information to establish the requiredform of intermediary pulse shape 14 needed to define a substantiallyrectangular switching window 50. The impulse response 30 of the grating11 required to convert the second signal pulses 4 can then be evaluated,and the required supestructured grating design itself derived using asuitable inverse grating design algorithm such as R. Feced, M. N.Zervas, and M. A. Muriel, “An efficient inverse scattering algorithm forthe design of non-uniform fibre Bragg gratings”, IEEE J. QuantumElectron., vol. 35, pp. 1105–1111, 1999.

In order to determine the impulse response 80 of the non-linear opticaldevice 12 (as a function of incident pulse power) it may be necessary touse an experimental pulse characterisation technique capable ofproviding both phase and amplitude information. Appropriate techniquesare described in B. C. Thomsen, P. Petropoulos, H. L. Offerhaus, D. J.Richardson, and J. D. Harvey, “Characterization of a 10 GHz harmonicallymode-locked erbium fibre ring laser using second harmonic generationfrequency resolved optical gating”, Technical Digest CLEO '99,Baltimore, 23–28 May 1999, paper CTuJ5. Further means to provide suchcharacterisation are found described in “Measuring ultrashort laserpulses in the time domain using frequency resolved optical gating” by R.Trebino, K W DeLong, D N Fittinghoff, J N Sweetser, M A Krumbugel, B ARichman, D J Kane in Review of Scientific Instruments, Vol. 68,3277–3295, (1997).

Also shown in FIG. 2 is a filter 21 that may be necessary for filteringthe output of the non-linear optical device 12. The filter 21 may be apolarising element such as a polarizer or a polarization beam splitter,a wavelength selective filter such as an optical fibre Bragg grating orthin film filter. When used in conjunction with a semiconductor opticalamplifier as shown in FIG. 2, the filter 21 is preferably a wavelengthselective filter.

FIG. 9 shows another embodiment of the invention in which the firstsignal 1 comprises a plurality of first signal pulses 15. The grating 11is defined by a grating impulse response 100. As shown in FIG. 10, theintermediary pulse shape 14 is defined by the convolution of the secondpulse shape 5 and the grating impulse response 100. The grating 11 beingsuch that the intermediary pulse shape 14 is a substantially rectangularpulse 95, and in which the apparatus has a tolerance to a variation inarrival time of the first pulse 15 at the first input port 8 and thesecond pulse 4 at the second input port 9 substantially equal to thewidth 101 of the substantially rectangular pulse 95 minus the width ofthe first signal pulse 15. The non-linear optical device 12 shown inFIG. 9 is a non-linear loop mirror (NOLM) 90 comprising dispersionshifted optical fibre (DSF) 91 and two couplers 92. The couplers arepreferably 3 dB optical fibre couplers. The DSF 91 is configured to havenormal dispersion at the first wavelength 3 and may have a length in theregion 1 m to 10,000 m depending on the amount of non-linearity present.The NOLM 90 may further comprise a polarization controller 93. The firstwavelength 3 is different from the second wavelength 6. The firstwavelength 3 is usually at a shorter wavelength than the second signalwavelength 6 for fibre based devices since: (a) it is often important tominimise group velocity induced walk-off between the first andintermediary signal pulses 15, 13 as they propagate through thenonlinear fibre and this is most readily achieved by arranging the twosignal wavelengths to lie roughly symmetrically about the zerodispersion wavelength of the fibre, and (b) the switched, or modulatedsignal 1 needs to lie in the normal dispersion regime in order to avoidthe impact of induced intensity noise due to soliton effects. The filter21 is preferably a wavelength selective filter, which may be a fibreBragg grating or a thin-film filter.

The tolerance to difference in arrival times, and thereby jitter, of thefirst and second optical signals 1 and 2 is determined by the widths 50and 101 of the substantially rectangular switching window 50 or thesubstantially rectangular pulse 95 respectively. It is envisaged thatusing suitably fabricated gratings 11 this tolerance could be as largeas 200 ps. This tolerance is suitable for high jitter tolerance opticalprocessing at data rates as low as 5 Gbit/s. At the other extreme weenvisage that the technology should also allow the generation ofintermediary pulses 13 with widths as short as 100 fs, making itsuitable for optical processing at data rates between 1 Tbit/s to atleast 10 Tbit/s. The technology should also allow processing atintermediate date rates.

FIG. 11 shows an embodiment in which the apparatus comprises an opticalswitch 111, the optical switch 111 being such that it comprises thenon-linear optical device 12. The optical switch 111 can be used in eachof the embodiments shown in FIGS. 1, 2 and 9. The non-linear opticaldevice 12 may comprise a lithium niobate channel waveguide, or a lithiumniobate planar waveguide. The non-linear optical device 12 may comprisea periodically poled lithium niobate channel waveguide or a periodicallypoled lithium niobate planar waveguide. The non-linear optical devicemay comprise a holey fibre, a poled-fibre, a potassium titanyl phosphate(KTP) or other crystalline waveguide, a periodically poled KTP or othercrystalline waveguide, a non-linear optical loop mirror, a Kerr gate, anoptical fibre, a non-linear amplifying loop mirror, or a non-linearoptical modulator.

FIG. 12 shows the cross-section of a holey fibre 120 which can beconfigured as the non-linear optical device 12 in the embodiments ofFIGS. 1, 2 and 9. The holey fibre 120 comprises a core 121 and acladding 122, in which the cladding 122 comprises a plurality of holes123 arranged around the core 122. The holey fibre 120 has a corediameter 123 of 2 μm. The core 121 and the cladding 122 are shownenlarged in the inset. The core 122 can have a diameter 123 between 2 μmand 10 μm. Preferably the core 122 has a diameter 123 less than 2 μm. Byholey fibre, we mean a fibre comprising longitudinally-extending holesthat may be twisted along the length of the fibre and we include similaror alternatively-named fibres such as microstructured fibres andphotonic band-gap fibres.

The holey fibre 120 is made from a single transparent material 124(discounting air as a constituent material). The transparent material124 is silica. Holey fibres can also be made from other forms ofsilicate glass (e.g Lead glasses such as Schott glasses SF57, SF58,SF59, or Bismuth oxide based glasses), or indeed any other form of glassincluding any compound glass (eg multi-component glasses such aschalcogenide glasses). Preferably, the glass would have a large thirdorder (Kerr) non-linear coefficient (>2.10⁻²⁰ m²/W), and low loss at theoperating wavelength of the non-linear optical device (<10 dB/m). Thematerial may also be a polymer, such as polymethyl methacrylate (PMMA)although any polymer with a significant second order (>0.01 pm/V), orthird order non-linear coefficient (>2.10⁻²⁰ m²/W) could be envisaged.The holey fibre 120 may also comprise a dopant in either the core 121 orthe cladding 122, or both. The dopant may comprise Ytterbium, Erbium,Neodymium, Praseodymium, Thulium, Samarium, Holmium Dysprosium, Tin,Germanium, Phosphorous, Aluminium, Boron, Antimony, Uranium, Gold,Silver, Bismuth, Lead, a transition metal, and a semiconductor.

FIG. 13 shows an apparatus comprising the holey fibre 120 and couplingmeans 131 for coupling optical energy into and out of the holey fibre120. The length of the holey fibre 120 can be between 0.1 m and 10 km.The dispersion can be normal or anamalous at the first signal wavelength3. In certain instances it is preferable for the dispersion to be normalat the first signal wavelength 3 and anomalous at the second signalwavelength 6 to minimise pulse walk-off. Normal dispersion for the firstsignal wavelength is advantageous in order to avoid soliton inducednoise effects. In other instances it may be desirable to have both thefirst and second signal wavelengths 3, 6 in the normal dispersion regimein order to avoid soliton-induced intensity noise effects.

FIG. 14 shows an apparatus comprising an interferometer 140 comprising afirst arm 141 and a second arm 142. The first arm 141 comprises thenon-linear optical device 12. The interferometer 140 is a Mach Zehnderinterferometer. The apparatus can also be constructed using a MichelsonInterferometer, a Sagnac interferometer or another configuration ofoptical interferometer.

FIG. 15 shows an apparatus configured as an optical demultiplexer 1500.The apparatus de-multiplexes the first signal 1 into a plurality oflower data rate signals 157. The apparatus comprises a clock generator151, couplers 152, a plurality of the non-linear optical devices (NOD)12 and the filters 21 interconnected by first optical fibres 153 andsecond optical fibres 154. The first and second optical fibres 153, 154have lengths configured such that the non-linear optical devices 12demultiplex the first optical signal 1 into different ones of the lowerdata rate signals 157. It is preferable that the clock generator 151 issynchronized to the first optical signal 1 utilizing a tap 150 which maybe an optical coupler. In the example shown, the clock generator 151will output a second optical signal 2 having a frequency four times lessthan the frequency of the first optical signal 1. The filters 21 havefilter output ports 155 that are shown connected to opticalcommunication lines 156 that may comprise at least one optical amplifier158.

The clock generator 151 may comprise a short-pulse generator. The shortpulse generator can be a mode-locked fibre laser, an activelymode-locked fibre laser, a generator comprising an electro-absorptionmodulator and a laser, a generator comprising an electro-optic modulatorand a laser, a gain-switched laser diode. The short-pulse generator mayalso comprise a means for pulse compression such as dispersioncompensator fibre, a chirped fibre Bragg grating, a dispersiondecreasing fibre, an optical amplifier, a Raman amplifier, an opticalswitch, an optical pulse compressor, or some combination of thesedevices.

FIG. 16 shows an apparatus configured as an optical multiplexer 1600.The apparatus multiplexes a plurality of first optical signals 1 into ahigher data rate signal 161. The apparatus comprises a multiplexer 160,an output port 162, and a plurality of optical fibres 161. The opticalfibres 161 have lengths configured such that the non-linear opticaldevices 12 multiplex the first optical signals 1 into the higher datarate signal 161 without cross-talk. It is preferable that the clockgenerator 151 is synchronized to at least one of the first opticalsignals 1 utilizing a tap 150 which may be an optical coupler. In theexample shown, the clock generator 151 will output a second opticalsignal 2 having a frequency four times greater than the frequency of thefirst optical signal 1. The apparatus may further comprise a wavelengthconverter for converting the wavelength of one or more of the firstoptical signals 1.

FIG. 17 shows an apparatus configured as an inverse multiplexer 1700.The inverse multiplexer demultiplexes a first optical signal 1 into aplurality of lower data rate signals 179 having different wavelengthsfor transmission through a wavelength division multiplexed (WDM) opticaltransmission line 172. This application has advantages fordemultiplexing a 40 Gb/s signal into four 10 Gb/s signals operating onfour wavelength channels in a WDM system. The optical transmission line172 may be between 100 m and a thousand kilometres in length, and maycomprise at least one optical amplifier 158. The demultiplexing stage1701 is similar to the apparatus shown in FIG. 15 except that the lowerdata rate signals 179 have different wavelengths. The apparatustherefore contains wavelength converters 170 configured such that thelower data rate signals 179 have different wavelengths from each other.The wavelength converters are shown connected to the second opticalfibres 154. They can also be connected to the first optical fibres 153or the filters 21. Alternatively, they be an integral part of thenon-linear optical devices 12. The wavelength converters 170 maycomprise a semiconductor amplifier, a NOLM, a dispersion shifted fibre,an optical fibre, a holey fibre, or a lithium niobate device.

The lower data rate signals 179 are multiplexed with a multiplexer 171into a wavelength division multiplexed signal 178 which is transmittedthrough a wavelength division multiplexed optical transmission line 172.

The multiplexing stage 1702 following the optical transmission line 172is similar to the apparatus of FIG. 16 except that it comprises awavelength division demultiplexer 173 that demultiplexes the lower datarate signals 179. The multiplexing stage 1702 may be exchanged with anelectronic multiplexer.

The inverse multiplexer 1700 may require dispersion compensators tobalance the overall group delays between the different wavelengthchannels propagating through the optical transmission line 172.Differential delays may also be required between the differentwavelength channels in order to reconstitute the first signal 1 properlyat the output 162. The differential delays may be introduced by takinggreat care in the optical path lengths in the demultiplexing stage 1701and the multiplexing stage 1702. Differential delays may also be addedelectronically at the receiver utilizing some form of patternrecognition on the received data.

FIG. 18 shows an apparatus comprising a control line 180 and a switch181. The control line 180 may be an optical control line 180 and theswitch 181 may be an optical switch. The switch 181 can also be locatedon the first optical input 8 or connected to the filter 21. Theapparatus is useful for turning the non-linear optical device 12 on andoff and can be used to provide intelligent routing of optical signalsthrough optical telecommunication networks. The apparatus shown in FIG.18 can be incorporated in any of the embodiments of the inventiondescribed above, for example in order to add intelligence into anoptical network.

FIG. 19 shows an apparatus configured as an optical multiplexer 1900with signal regeneration capability. The apparatus multiplexes aplurality of first optical signals 1 into a higher data rate signal 193and can serve to reduce both timing and amplitude jitter on the firstoptical signals 1. The apparatus comprises a multiplexer 190 whichserves to interleave the incoming first optical signals 1 into a higherdata rate signal 191 without introducing substantial cross talk. To dothis reliably means that the individual first optical signals 1 need tobe mutually synchronised and in certain instances some form of dynamiccontrol may be required to ensure this. The multiplexer 190 might be anarray of optical fibre couplers, or a planar lightwave circuit withappropriate delays for the different optical paths through the system.It is preferable that the clock generator 151 is synchronized to atleast one of the first optical signals 1 utilizing a tap 150 which maybe an optical coupler. In the example shown, the clock generator 151will output a second optical signal 2 having a frequency four timesgreater than the frequency of the first optical signal 1. The opticalsignal 191 is then converted to a high frequency optical data signal 192in which the individual pulses have the intermediary pulse shape 194required to obtain a square switching response from the non-linearoptical device 12. The optical signal 192 is then input to thenon-linear optical device 12. The square temporal switching window 195of the non-linear device 12, and the non-linear intensity dependenttransmission response 490 of the non-linear optical device (see FIG.49), then serve to reduce any timing, or amplitude noise associated withincoming relatively-lower data rate signals 1, providing a less-noisyinterleaved signal 193.

FIG. 20 shows an apparatus configured as an optical signal multiplexer2000 with signal regeneration capabilities. The apparatus multiplexes aplurality of first optical signals 1 each with a different wavelength(e.g. separate WDM channels) into a higher data rate signal 193 and canserve to reduce both timing and amplitude jitter on the first opticalsignals 1. Wavelength converters 170 are used to wavelength convert eachof the incoming signals to have the same wavelength at the input to themultiplexer 190, which serves to interleave the incoming first opticalsignals into a higher data rate signal 191 without introducingsubstantial cross talk. To do this reliably means that the individualfirst optical signals need to be mutually synchronised and in certaininstances some form of dynamic control may be required to ensure this.The multiplexer 190 might be an array of optical fibre couplers, or aplanar lightwave circuit with appropriate delay for the differentoptical paths through the system. It is preferable that the clockgenerator 151 is synchronized to at least one of the first opticalsignals 1 utilizing a tap 150 which may be an optical coupler. In theexample shown, the clock generator 151 will output a second opticalsignal 2 having a frequency four times greater than the frequency of thefirst optical signal 1. The optical signal 191 is then converted to ahigh frequency optical data signal 192 in which the individual pulseshave the intermediary pulse shape 179 required to obtain a squareswitching response from the switch. This optical signal is then input tothe nonlinear optical device 12. The square temporal switching window195 of the nonlinear device 12, and the periodic nonlinear intensitydependent response 490 of the switch (see FIG. 49), then serve to reduceany timing, or amplitude jitter associated with incomingrelatively-lower data rate signals 1, providing a less-noisy interleavedsignal 193.

The following examples describe the use of superstructured fibre Bragggratings (SSFBGs) to convert the output of an actively mode-locked, 2.5ps fibre laser, a reliable source of short pulses of a well-definedsoliton shape, to 20 ps rectangular pulses. These pulses are then usedto control the operation of two sorts of non-linear switch. Highquality, ˜15–20 ps rectangular switching windows are obtained, providing+/−7 ps, 15 ps timing jitter tolerance, in switches based on both theKerr effect in dispersion shifted fibre (DSF), and on four-wave mixingin a semiconductor amplifier (SOA).

SuperStructured Fibre Bragg Gratings (SSFBGs) can be considered andemployed as spectral filters of controllable phase and amplitude. Theterm SSFBG refers to a fibre Bragg grating (FBG) whose refractive indexprofile is not uniform in amplitude and/or phase along its length. Forease of discussion, we restrict the following discussion to the weakgrating limit in which the relative changes of its refractive index aresmall enough to allow the incident light to penetrate the full devicelength, such that the whole grating contributes equally to the reflectedsignal. However, it should be appreciated that due to recent advances ingrating design algorithms the general principles outlined below can nowbe readily applied to the high reflectivity, non-Fourier designlimit—see for example R. Feced, M. N. Zervas, and M. A. Muriel, “Anefficient inverse scattering algorithm for the design of nonuniformfibre Bragg gratings”, IEEE J. Quantum Electron., vol. 35, pp.1105–1111, 1999. For a weak SSFBG the wavevector response F(κ) is givenby the Fourier transform of the spatial refractive index modulationprofile A(x) used to write the grating, where κ is the wavevector, whichis proportional to the frequency ω, i.e. $\begin{matrix}{{F(\kappa)} = {\frac{1}{2\;\pi}{\int_{- \infty}^{+ \infty}{{A(x)}{\mathbb{e}}^{j\;\kappa\; x}{{\mathbb{d}x}.}}}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

The impulse response h(t) of a fibre grating is given by the inverseFourier transform of its frequency response H(ω) $\begin{matrix}{{h(t)} = {\int_{- \infty}^{+ \infty}{{H(\omega)}\;{\mathbb{e}}^{{- j}\;\omega\; t}{{\mathbb{d}\omega}.}}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

This equation is true for any fibre grating—ie for both weak and strongSSFBG's.

It follows that the impulse response h(t) of a weak FBG is a pulse ofthe same temporal profile as the spatial modulation profile A(x) of thegrating (with an appropriate conversion from the space to time frame viat=2x·n/c, where n is the refractive index of the fibre core). The(reflected) optical response y(t) of the grating to a pulse of finitetime duration x(t) is given by the convolution of the input signal withthe grating's impulse response i.e.y(t)=x(t)*h(t).  (Eq. 3)

Alternatively, as expressed in the frequency domain, the reflectedsignal Y(ω) is the product of the incident signal X(ω) with H(ω)Y(ω)=X(ω)H(ω)  (Eq. 4)

where Y(ω) and X(ω) imply the Fourier transforms of y(t) and x(t)respectively. Thus it can be appreciated that, for well-specified inputand target output pulse forms, one can (subject to the usual laws ofcausality and the limits of fibre Bragg grating FBG technology) designand fabricate an FBG to perform the required shaping operation. SSFBGsare attractive for many pulse shaping applications since they offer allthe advantages associated with fibre components, such as readyintegration into fibre systems and low coupling losses. Moreover, theyare potentially low-cost devices.

Advances in the fabrication of FBGs now allow the fabrication ofgratings with truly complicated amplitude and phase characteristics,greatly extending the potential and range of applications of theapproach.

The following experiments demonstrate the fabrication and use of a trulycomplex superstructure grating designed to transform short opticalpulses (2.5 ps at 10 GHz) into a corresponding train of 20 psrectangular pulses. The results achieved highlight the quality of the“continuous grating writing” technique and establish the superstructuretechnique as a viable means for achieving a broader range of pulseshaping functions than had generally been considered technologicallyfeasible.

With reference to FIGS. 22 and 23, since the pulse shaping provided bythe grating 11 is a purely passive-filtering process it is necessary toprovide a well-defined input pulse form 230 to filter and therebyreliably re-shape. In the following discussion, “input pulses” refers tothe second signal pulses 4, and “output pulses, target pulses and outputwaveforms” refers to the intermediary pulses 13. Input pulses weregenerated using an actively mode-locked erbium doped fibre ring lasercontaining anomalously dispersive fibre, which naturally generateshigh-quality optical solitons by virtue of its principle of operation.The target output waveform 231 was a rectangular pulse with 20 psduration. This particular pulse duration was chosen to ensure anadequate number of spectral features could be accommodated within thefinite available spectral bandwidth defined by the input pulse form 230and the SSFBG reflectivity bandwidth. (The input pulse duration was 2.5ps (full width at half maximum—FWHM) and the SSFBG bandwidth wasrestricted to 6 nm, which represents the −40 dB spectral bandwidth forsuch pulses).

FIG. 22 shows the input pulse spectrum 220 and the output pulse spectrum221 associated with the choice of input and output pulse forms 230, 231.The spectrum of an idealized rectangular pulse is a sinc function, whichexhibits lobed features of alternating optical phase. For the particularchoice of relative pulse durations, 13 spectral lobes can be accomodatedwithin the available 6 nm spectral bandwidth. By retaining a significantnumber of features (and associated broadband spectral components) fastrise times 232 and fall times 233 can be obtained on the output pulse231. The spectral truncation gives rise to the development of a ‘ringingstructure’ 234 close to the rectangular pulse edges in the time domain(Gibbs phenomenon). The design of the grating 11 seeks to minimize theseeffects by apodizing the output spectrum 221 using a Gaussian profile,so that the targeted signal spectrum 221 follows the mathematicalspecification: $\begin{matrix}{{Y(\omega)} = {\frac{\sin\left( {p\;\omega} \right)}{p\;\omega} \cdot {{\mathbb{e}}^{- {({a\;\omega})}^{2}}.}}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

The factor p in this equation determines the spectral width, and in thiscase was set to 9.87 THz⁻¹. The expense of introducing apodization is aslight increase in the rise and fall times of the pulses 232, 233. Theapodization factor a was kept fairly small, namely 0.55 THz⁻¹, to give asatisfactory trade-off between the two undesirable effects. The 10–90%rise/fall times of the target pulses 231 were 1.6 ps and the relativeripple depth 235 as shown in FIG. 23 was 0.03. The corresponding figuresfor non-apodized pulses are rise/fall times of 1.0 ps and a relativeripple depth of 0.24. Note here that we define the relative ripple depthas the ratio of the difference between the highest and lowest intensitypoints of the ripple at the top of the rectangular pulses 231 to themaximum intensity of the pulses.

If the input signal were an impulse, then the modulation profile of theSSFBG and the temporal-profile of the output signal would be identical.However, because of the finite duration of the input pulses 230, boththe shapes of the target signal 231 y(t) (or Y(ω)) and the input signal230 x(t) (or X(ω)) need to be taken into account during the SSFBG designprocess. The required response of the grating 11 in the frequency domainH(ω) can be calculated from Eqn. 4 as the quotient of the outputfrequency spectrum 221 Y(ω) (desired) to the input frequency spectrum220 of the ideal soliton pulses 230 X(ω). A plot of the desiredreflectivity response 240 is shown in FIG. 24 by the dashed line. Thecorresponding refractive index superstructure 250 of the grating 11required to achieve the response 240 is obtained from inverting Eqn. 1and is shown in FIG. 25. The detailed structure of the desiredreflectivity response 240 highlights the precision required of thegrating writing process. Note that the sections of negative inducedindex 251 are achieved by putting additional discrete positive andnegative phase shifts 241, 242 into the structure 250 such that apositive index change from the base line level can be used over theentire length of the grating 11—see for example M. Ibsen, M. K. Durkin,M. J. Cole, and R. I. Laming, “Optimised rectangular passband fibreBragg grating filter with in-band flat group delay response”, Electron.Lett., vol. 34, pp. 800–802, 1998. The length of the grating in the timedomain was t=100 ps, corresponding to a grating length of 0.5·t·c/n=10.3 mm.

The sensitivity of the shaping operation to a variety of non-idealoptical excitation conditions and grating fabrication defects was thentested numerically. By non-ideal we mean differing in some regard fromthe design input pulse form 230.

Firstly, the effect of using pulses with a soliton pulse shape, but aduration differing from the duration used for the SSFBG design 250 wasinvestigated. FIG. 27 shows the intensity 271, 272, 273 for solitonpulses of widths 2.0 ps, 2.3 ps, and 3.0 ps respectively. The dashedline is the intensity 231 representing the intensity of the 2.5 pssoliton pulse input described with reference to FIG. 23. The analysisshowed that small (+/−20%) departures of the pulse width from the ideal2.5 ps affected the trade-off between ripple strength and rise/falltimes.

Specifically, the ringing structure on the filtered pulses becomes moredominant for shorter pulses, whereas for wider pulses the rising andfalling edges of the output pulses become less sharp and they begin tolose their flat-top nature. This behaviour can be understood byvisualizing the signals in the frequency domain. The result of usingshorter pulses, which exhibit a wider bandwidth, is to partially cancelout the apodization. Similarly, using broader pulses, which exhibit anarrower bandwidth, imposes additional apodization on the output pulses.

Secondly, the use of pulses with the desired width but with pulse shapesthat differ significantly from the desired transform limited solitonform 230 was investigated. The pulses used to generate the responsesshown in FIG. 28 correspond to 2.5 ps linearly chirped soliton pulseswith a different chirp parameter C as defined below: $\begin{matrix}{{x(t)} = {\sec\mspace{11mu}{{h\left( \frac{1.763\; t}{\Delta\; T} \right)} \cdot {\mathbb{e}}^{- \frac{{j\; \cdot 1.554}\; C\; t^{2}}{\Delta\; T^{2}}}}}} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$

where ΔT is the full width half maximum FWHM of the pulses. FIG. 28shows the output pulses 281, 282, 283 corresponding to 2.5 ps chirpedsoliton pulses with C=0.1, TBP=0.342; C=0.2, TBP=0.420; and C=0.45,TBP=0.518, respectively. The dashed line is the intensity 231representing the intensity of the 2.5 ps soliton pulse input describedwith reference to FIG. 23. The performance of the filter is seen to bereasonably good for relatively small amounts of chirp (C≦0.1), howeverfor more extreme chirps the pulse deformation becomes more severe andspikes begin to develop at the leading and trailing edges.

FIG. 29 shows the output pulses 291, 292 corresponding totransform-limited Gaussian input pulses of widths 2.5 ps and 3.55 psrespectively. The dashed line is the intensity 231 representing theintensity of the 2.5 ps soliton pulse input described with reference toFIG. 23. Gaussian pulses are characterized by a wider spectrum thanthose of soliton pulses of the same FWHM. For 2.5 ps Gaussian inputpulses the deformation of the filtered output is quite significant andis characterized by the formation of sharp spikes on the pulse edges. Bycontrast the response 292 closely resembles the idealized case 231 whenGaussian pulses with the same 3 dB spectral width as a 2.5 ps solitonare used: the pulse duration in this instance is 3.55 ps . There ishowever a slight compromise in the rise and fall times, and the pulsesexhibit slight spikes close to the edges.

The SSFBG used in the analysis had a central reflecting wavelength of1550 nm. FIG. 30 shows the effect of a mismatch between the centralwavelength of the SSFBG to that of the incoming 2.5 ps soliton pulses.Pulse 301 corresponds to a mismatch of 0.4 nm, pulse 302 to a mismatchof 0.7 nm, and pulse 303 to a mismatch of 1.4 nm. The dashed line is theintensity 231 representing the intensity of the 2.5 ps soliton pulseinput described with reference to FIG. 23 ie with no wavelengthmismatch. Significant distortion of the pulses only becomes evident forwavelength mismatches greater than ˜0.3 nm, and again manifests itselfas the formation of dominant spikes at the pulse edges. Moreover, theintensity of the central part of the pulse is decreased. For moreextreme mismatch cases (e.g. 1.5 nm) the spike formation is so severethat the reflected waveform effectively divides into two distinctpulses. It is therefore preferable that the mismatch is less than 0.3 nmfor an SSFBG having a central reflecting wavelength in the range fromaround 1500 nm to around 1650 nm.

Finally, the effect that potential imperfections in the SSFBG structurecould have on the shaping action was investigated numerically. Gratingimperfections can arise either due to errors associated with the UVexposure itself (e.g. laser power fluctuations, phase mask errors), orfrom small variations in the fibre core diameter. Such imperfectionsmanifest themselves as both phase and amplitude errors in the complexSSFBG superstructure function. It is difficult to estimate reliably thecontribution of these imperfections. However, in order to gain aninsight, their effect was simulated by adding noise (both phase andamplitude) to the SSFBG refractive index profile 250. For the purpose ofthe calculations, the phase and amplitude noise was assumed to beindependent of each other, and to be randomly distributed along thegrating length subject to a normal distribution with well defined meanand variance. The local values of the ideal grating superstructurefunction 250 were mathematically modified by:A _(n)(x)=|A ₀(x)|·n ₁(x)·e ^(−j(arg(A) ⁰ ^((x))+2πn) ² ^((x)))  (Eq. 7)where A₀(x) is the ideal superstructure function, and n₁(x) and n₂(x)are the random amplitude and phase noise parameters respectively. Thetemporal shapes of two extreme cases of distorted pulses 321, 322 areshown in FIG. 32 where the noise was added from a computer-generatednoise function. The value of the phase noise parameter n₂(x) used forboth the pulses 321, 322 had a mean value of 0 and a standard deviationof 0.04, whereas the amplitude noise n₁(x) for both the pulses 321, 322was ascribed a mean value of 1 and a standard deviation of 0.02. Notethat these values of standard deviation should be considered asextremely large for such a short FBG and the “continuous gratingwriting” technique employed. However, even with such a large noisecontribution it can be seen that its effects are still somewhat minimal,further confirming the robustness of the shaping action.

Examining the various individual plots presented in FIGS. 27 to 30 and32, it is clear that the shaping mechanism is reasonably robust and notparticularly sensitive to the precise pulse excitation parameters, orsmall deviations in grating design. Indeed all of the estimatedtolerances are well within readily achievable experimental limits, asdemonstrated below.

The experimental set up used is shown in FIG. 31. Anall-polarization-maintaining harmonically mode-locked erbium fibre ringlaser EFRL 310 operating at a repetition rate of 10 GHz was used togenerate 2.5 ps soliton pulses 317 which could be monitored by aspectrum analyser 311 via a coupler 316. The grating 11 was astrain-tunable SSFBG 315 which output the reflected pulses 318 via acirculator 312. These were amplified by an erbium doped fibre amplifier(EDFA) 313 to produce a shaped output 314. For further details of thedesign of the EFRL see B. C. Thomsen, P. Petropoulos, H. L. Offerhaus,D. J. Richardson, and J. D. Harvey, “Characterization of a 10 GHzharmonically mode-locked erbium fibre ring laser using second harmonicgeneration frequency resolved optical gating”, Technical Digest CLEO'99, Baltimore, 23–28 May 1999, paper CTuJ5. The central wavelength ofthe laser 310 was tunable through the use of an intra-cavity band-passfilter (not shown). The inset of FIG. 33 shows the optical spectrum 330of the pulses—these have a 3 dB bandwidth of 1.0 nm. The input spectrumis composed of discrete, essentially infinitely narrow spectral lines331 separated by ˜0.08 nm corresponding to the signal repetitionrate—these lines are clearly resolved in this high-resolution scan(resolution: ˜25 pm). The corresponding autocorrelation trace of theinput pulses is shown in FIG. 34, which compares the measuredautocorrelation trace 341 of the 2.5 ps soliton pulses 317, the measuredautocorrelation trace 342 of the reflected pulses 318, with thecalculated autocorrelation function 343 (dashed line) of the rectangularpulses 231. It is seen that the soliton pulses have a FWHM of 2.5 ps.This yields an estimated time-bandwidth product (TBP) of ˜0.32 givingconfidence that the pulses are indeed close to transform-limitedsolitons. The pulses were launched through a short length of fibre ontothe SSFBG 315 via the 3-port optical circulator 312. The resulting pulseshaping effects upon reflection from the SSFBG 315 were investigated inthe time and frequency domains at the circulator output port. The SSFBG315 was mounted on a rig to allow for fine strain-tuning of its centralwavelength relative to that of the laser 310. The SSFBG 315 was writtenin a 0.12NA germanosilicate fibre with a 100 mW, 244 nm CW UV-sourceusing the “continuous grating fabrication technique” as described inU.S. Pat. No. 6,072,926, which is hereby incorporated herein byreference. The fabrication technique effectively writes grating plane bygrating plane, and apodization is obtained by dephasing one gratingperiod with respect to the next one, or in other words by filling up thegaps between the grating planes to effectively reduce the index depth n,at the same time keeping the average refractive index n_(ave) constant.To obtain full control of the apodization, the gratings are fabricatedin the regime, where the index changes in a linear fashion with fluence.An interferometer is used to monitor the position of the fibre duringwriting to ensure that the individual grating planes are written with aposition accuracy of ˜1.0 nm. The peak reflectivity of the grating waskept relatively low (˜10%) to ensure that operation within the Fourierlimit. Based on this figure, the energy efficiency of the whole pulseshaping system was calculated to be ˜3.5%. A plot of the amplitude andtime delay response 243, 260 of the resulting SSFBG 315 is shown inFIGS. 24 and 26 respectively, as measured with an optical networkanalyzer. In FIG. 24, the dashed line shows the calculated spectralresponse 240 of the structure designed. The agreement with theexperimentally measured amplitude response 243 is seen to be excellent.Direct evidence for the discrete phase jumps between the individualreflectivity lobes of the grating is given by the observation of sharpfeatures in the time delay response 260 at the lobe edges as seen inFIG. 26. The flatness of the time delay response 260 within the lobepass band, albeit limited by system measurement noise/resolution, alsoprovides a good indication of a uniform phase response across the mainbody of the individual lobes as desired.

The measured power spectrum 332 of the reflected pulses 318 is shown inFIG. 33. This is compared to the spectrum 333 of the single rectangularpulse 231 expected from the design procedure. There is a very goodagreement between the envelopes of the two spectra even at levels ˜25 dBbelow the main spectral peak. (The distinct spectral lines of theexperimental trace arise from the high repetition rate of the signal,which was not taken into account on the calculated filter response andare readily resolved by the spectrum analyzer). The temporal shape ofthe reflected pulses 318 was initially evaluated using anautocorrelator. The intensity autocorrelation function of a rectangularpulse of duration T is a triangular pulse of total duration 2 T. FIG. 34shows the measured autocorrelation trace 342 of the reflected pulses318, the calculated autocorrelation function 343 of the targetedwaveform 318, and an autocorrelation trace 341 of the input pulses 317.The shaping action of the SSFBG can easily be appreciated. The fullwidth of the triangular autocorrelation function 342 is approximately 40ps as expected for a 20 ps pulse form.

To establish the quality of the shaping more directly measurements wereconducted using an optical sampling oscilloscope. The system used anelectroabsorption modulator and an electronically driven delay circuitto sample the optical signal at delayed times relative to the fixed RFdrive to the laser—for a fuller explanation of the technique, see A. D.Ellis, J. K. Lucek, D. Pitcher, D. G. Moodie, and D. Cotter, “Full 10×10Gbit/s OTDM data generation and demultiplexing using electroabsorptionmodulators”, Electron. Lett., vol. 34, pp. 1766–1767, 1998.

FIG. 35 shows measured optical sampling oscilloscope traces 350 of theinput pulses 317 and FIG. 36 shows measured optical samplingoscilloscope traces 360 of the reflected pulses 318. The resolution ofthe oscilloscope was approximately 7 ps as determined by measurements ofthe incident 2.5 ps pulse forms shown in FIG. 35. The measurements showthat the reflected pulse 318 has a substantially rectangular pulseshape. An accurate estimate of the rise and fall times on the pulse islimited by the 7 ps temporal resolution of the measurement apparatus.There appears to be slight amplitude variation (approximately 5–10%)across the top of the pulses 360. At this stage it is not yetestablished whether this variation is due to grating imperfections, oris an artifact of the optical sampling scope set up. Nevertheless, themain targets of the shaping operation, i.e. the generation of an almostflat top and sharp edges, are clearly demonstrated.

In additional experiments the central wavelength of the laser 310 wasdetuned relative to that of the SSFBG 315, and filtered signal 318 wasdiagnosed using the optical sampling oscilloscope and an opticalspectrum analyzer. The results of these measurements are summarized inFIG. 37, which shows the measured power spectra 370, 371 for wavelengthmismatches of 0.4 nm and 1.4 nm respectively, and in FIG. 38 which showsoptical sampling oscilloscope traces 380, 381 for the wavelengthmismatches of 0.4 nm and 1.4 nm respectively. The results shown in FIGS.37 and 38 should be compared to the numerical calculations presented inFIG. 30 (taking into account of course the limited resolution of theoptical oscilloscope). The two cases shown in FIGS. 37 and 38 are for anincoming signal of central wavelength 0.4 and 1.4 nm away from thecentral wavelength of the SSFBG 315 respectively. In both cases, theinput pulses 317 were transform-limited solitons of 2.5 ps duration. Thegeneral behaviour predicted in FIG. 30 is confirmed here, with thecentral part of the pulse decreasing, until the pulse splits into twoparts.

With reference to FIG. 31, Bit-Error-Rate (BER) measurements wereperformed at 10 Gbit/s using the rectangular pulses 318. For thesemeasurements, the laser 310 was operated at the central wavelength ofthe SSFBG 315 and produced transform-limited 2.5 ps soliton pulses 317.The 10 GHz laser signal 317 was modulated using a 2³¹−1 pseudorandom bitsequence before being coupled onto the SSFBG 315. The reflected signal318 was detected using a commercial 10 Gbit/s RZ photoreceiver and fedto the BER tester. The BER measurements are summarized in FIG. 39. Curve391 shows the BER measurements using the rectangular pulses 318, andcurve 392 shows BER measurements taken with the soliton pulses 317without passing the soliton pulses 317 through the SSFBG 315 (so calledback-to-back measurements). The results indicate that essentially errorfree operation was readily achieved down to the 10⁻¹¹ level, with only aslight (<0.5 dB) power penalty relative to the back-to-backmeasurements.

The utility of producing high-quality, soliton to rectangular pulseconversion using reflection from a complex superstructure grating withan appropriately designed amplitude and phase response has clearly beendemonstrated and the achieved performance are in good agreement withtheory. In addition, the tolerance of the proposed scheme to variousnon-ideal excitation conditions, and to random grating writing errorshave been demonstrated to be reasonably robust on both counts. Theresults highlight the capability of advanced grating writing technologyfor use in pulse shaping applications within the communications arena.

The experimental set up shown in FIG. 40 was used to demonstrate the useof the SSFBG 315 described above in non-linear switching applications.The set up comprises the 2.5 ps, 10 GHz, regeneratively mode lockederbium fibre ring laser (EFRL) 310, 50:50 couplers 401, erbium dopedfibre amplifiers (EDFA) 405, a modulator 403 driven by a patterngenerator 404, a circulator 406, the grating 315, polarizationcontrollers 407, 1 km of dispersion shifted fibre 408 having a zerodispersion wavelength of 1550 nm, 1 km of dispersion shifted fibre 409having a zero dispersion wavelength of 1554 nm, an output port 410,diagnostics 411, a tuneable delay line 412, and a continuous wave laser413.

The 2.5 ps 10 GHz solitons 317 from the EFRL 310 had a wavelength of1557 nm. The solitons 317 were separated into a first and a secondchannel 415 and 416. The first channel was modulated by the modulator403 driven by the 1 to 10 GHz pattern generator to provide apseudorandom data sequence 417 of 2.5 ps pulses at 2.5 Gbit/s. Thesepulses were then fed onto the pulse-shaping SSFBG 315 which wasfabricated with the correct phase and amplitude reflectivity profile toconvert the 2.5 ps solitons into 20 ps rectangular pulses 418.

The second channel 416 was first amplified and then fed to the controlport of a dual-wavelength NOLM 419, which was employed as a wavelengthconverter. The NOLM 419 incorporated a 1 km long dispersion shiftedfibre DSF 408 with a zero dispersion wavelength of λ₀=1550 nm. The NOLM419 acted as a non-linear switch that enabled the output of acontinuous-wave DFB laser 413 operating at 1544 nm to be modulated usingthe 1557 nm control pulses 317. By appropriately setting the loss andpolarization of light within the NOLM 419, and filtering out the 1557 nmcontrol pulses at the output of the NOLM 419, a 10 GHz train ofhigh-quality, 3.5 ps pulses 420 at 1544 nm was obtained. Importantly,for this demonstration this 10 GHz wavelength-shifted pulse train 420was synchronized to the 2.5 Gbit/s data stream 418 generated within thefirst channel 415.

FIG. 41 shows the measured triangular autocorrelation profiles 4100,4111, 4112 of the pulses 418, 417 and 420 respectively. FIG. 42 showsthe optical spectra 4200, 4210 of the pulses 418 and 420 respectively.Having generated these two synchronized pulse streams 418, 420, at twodifferent wavelengths and two different pulse repetition frequencies,the characteristics were measured of the NOLM 421 when controlled by the2.5 Gbit/s, 20 ps 1557 nm rectangular pulses 418 and the 2.5 ps 1557 nmsoliton pulses 317 respectively (ie with SSFBG 315 and without the SSFBG315 respectively).

For certain implementation of the jitter tolerant switch it is necessaryto control the polarisation state of the pulses incident on the SSFBG315 since it is possible to get a different impulse response fororthogonal polarisation components if the fibre used to manufacture theSSFBG is birefringent (either inherently, or due to the grating writingprocess), and this can degrade the performance of the switch. Usuallythis achieved by placing a polarisation controller somewhere in theoptical path to the SSFBG 315. Also note that often the response of thenon-linear switch itself is polarisation dependent. In certain instanceit is desirable for signals 1 and 2 (here represented by signals 418 and420) to be co-polarised when incident to the switch, in other instancesit is preferable that they are cross-polarised. Again additionalpolarisation controllers may be required within the circuit to ensurethat suitable polarisation alignment can be achieved.

FIG. 49 shows the transmission characteristics 490 of a typicalfibre-based non-linear optical device as a function of peak intensity ofthe control signal that controls the switching. The characteristic 490has a nonlinear intensity transmission response with a first operatingrange 491 over which the transmission changes little with peakintensity, and a second operating range 492 over which the transmissionremains low for a substantial peak intensity range. Such acharacteristic can provide amplitude noise suppression and opticalthresholding when used as the non-linear optical device 12 particularlywhen a rectangular control pulse is utilized that operates thenon-linear optical device 12 between the first and the second operatingranges 491, 492.

The first switch investigated was the dual-wavelength NOLM 421. In thisinstance however, the control signal was now a data-modulated signal417, and the signal to be switched was a 10 GHz train of 3.5 ps 1544 nmoptical pulses 420. The system thus operated in this instance as anall-optical modulator in this configuration and which for convenience issometimes referred to as an M-NOLM. Note that by reversing the input andcontrol signals 417 and 420 the system can be re-configured to act as anall-optical demultiplexer. The 1544 nm pulse train 420 incident to theswitch was first passed through a tunable delay line 412 to allow therelative arrival time of the 1544 nm pulses 420 relative to therectangular control pulses 418 to be adjusted. By adjusting andmeasuring this relative arrival time delay and monitoring the loopoutput 410 at 1544 nm, (for a suitable control pulse power), we wereable to determine the switching window of the device and to establishits sensitivity to timing-jitter.

FIG. 43 shows the theoretically predicted switching window 430 and theexperimentally observed switching window 432 of the NOLM 421 driven withthe 20 ps rectangular pulses 418. These are compared with thetheoretically predicted switching window 431 and the experimentallyobserved switching window 433 of the NOLM 421 driven with the 2.5 pscontrol pulses 417 (ie without the SSFBG 315 in place). A goodrectangular switching characteristic 430 with a 3 dB width of 20 ps isobtained using the rectangular control pulses as opposed to a value of 4ps when driving the switch without the SSFBG 315. Note the slightasymmetry in the switching window that is both predicted theoreticallyand observed experimentally. This arises from pulse walk-off effectsbetween the pump and probe beams within the NOLM 421. The effect thoughis small since the dispersion shifted fibre 409 used in the NOLM 421 hada length of only 1 km, a zero-dispersion wavelength of 1554 nm, and adispersion slope of 0.07 ps/nm²-km. These results show that we canexpect to achieve around 5 times greater tolerance to timing-jitter byconverting the control pulses to rectangular pulses in this instance.

In order to confirm the system impact of using rectangular controlpulses bit error rate BER measurements were performed on the NOLM 421switch performance. These results are summarized in FIG. 44, which showsthe BER 440, 441 versus time delay 442 when the NOLM 421 was driven bythe rectangular pulses 418 and by the soliton pulses 417. The time delay442 was varied with the tuneable delay line 412. Error-free,penalty-free performance was readily achieved over a +/−7 ps delay rangefor the rectangular pulse 418 driven NOLM 421 versus a +/−1 ps range forthe NOLM 421 driven directly with the 2.5 ps laser pulses 417. Nosignificant power penalties on BER for either the NOLM 419 or the NOLM421 were observed.

FIG. 45 shows the apparatus of FIG. 40 with the NOLM 421 replaced by asemiconductor amplifier SOA 451 which employs four-wave mixing as theoptical switching mechanism. The experimental set up was essentially thesame as that used for the NOLM 421. However it is to be appreciated thatthe optimum switching powers required for the SOA 451 based scheme(approximately 7 dBm average power at 10 Gbit/s) were substantiallylower than those required for the NOLM 421 (approximately 15 dBm at 2.5Gbit/s). Note also that we had to change the wavelength of the 10 GHzswitching pulses from 1544 nm to 1550 nm to achieve an adequate phasematching condition and sufficient switched power. A demultiplexed fourwave mixed signal 460 is observed at the wavelength of 1543 nm, as shownin the SOA output spectrum 461 of FIG. 46. FIG. 47 shows theexperimentally measured switching window 470. As with the NOLM 421 anexcellent rectangular switching window is obtained, allowing for timingjitter tolerance of +/−7 ps.

The above experiments demonstrate that SSFBGs can be used to reliablyre-shape ultrashort optical pulses in order to provide more optimal andjitter-tolerant operation of non-linear optical switches based on bothfibre and semiconductor non-linear components. This approach isparticularly attractive for use with SOA based switching devices forwhich there is no ready way of shaping the switching window other thanthrough direct control of the pulse shape.

The SSFBG approach represents an extremely powerful and flexible way ofmanipulating the temporal characteristics of pulses and that it couldplay an important role in future high-speed, high capacity opticalcommunication systems and networks.

It is to be appreciated that the embodiments of the invention describedabove with reference to the accompanying drawings have been given by wayof example only and that modifications and additional components may beprovided to enhance the performance of the apparatus. In particular,planar waveguide components may advantageously replace fibre componentsin many of the embodiments and in particular with reference to FIGS. 15,16, 17, 19 and 20.

The present invention extends to the above mentioned features takensingularly or in any combination.

1. Apparatus for providing timing jitter tolerant optical modulation ofa first signal by a second signal, the first signal having a firstwavelength, the second signal comprising a plurality of second signalpulses having a second pulse shape and a second wavelength, theapparatus comprising a first signal input port, a second signal inputport, a coupler, a grating and a non-linear optical device, theapparatus being configured to direct the second signal at the secondsignal input port to the non-linear optical device via the coupler andthe grating, and to direct the first signal at the first signal inputport to the non-linear optical device; the grating comprising asuperstructured fibre Bragg grating that converts the second signalpulses into intermediary pulses each having an intermediary pulse shape;the intermediary pulse shape being such that it provides a switchingwindow within the non-linear optical device.
 2. The apparatus of claim1, and wherein the first signal comprises a plurality of first signalpulses, the grating is defined by a grating impulse response, theintermediary pulse shape is defined by the convolution of the secondpulse shape and the grating impulse response, and the switching windowis a substantially rectangular switching window which provides toleranceto a variation in arrival time of the first signal pulse at the firstinput port and the second signal pulse at the second input portsubstantially equal to the width of the substantially rectangularswitching window.
 3. The apparatus of claim 1, and wherein the firstsignal comprises a plurality of first signal pulses, the grating isdefined by a grating impulse response, the intermediary pulse shape isdefined by the convolution of the second pulse shape and the gratingimpulse response, the grating being such that the intermediary pulseshape is a substantially rectangular pulse, and wherein the apparatushas a tolerance to a variation in arrival time of the first pulse at thefirst input port and the pulse at the second input port substantiallyequal to the width of the substantially rectangular pulse minus thewidth of the first signal pulse.
 4. The apparatus of claim 1, andwherein which the coupler is one of a circulator or an optical fibrecoupler.
 5. The apparatus of claim 1, and further comprising an opticalswitch, the optical switch comprising the non-linear optical device. 6.The apparatus of claim 1, and further wherein the non-linear opticaldevice is a holey fibre.
 7. The apparatus of claim 6, and wherein theholey fibre comprises glass, and further wherein the glass is one of asilica, a silicate glass, or a compound glass.
 8. The apparatus of claim6, and wherein the holey fibre comprises a polymer.
 9. The apparatus ofclaim 6, and wherein the holey fibre comprises a core and a cladding,the cladding comprises a plurality of holes arranged around the core,and the core has a diameter less than 10 um.
 10. The apparatus of claim9, and wherein the core has a diameter less than 5 um.
 11. The apparatusof claim 9, and wherein the core has a diameter less than 2 um.
 12. Theapparatus of claim 6, and wherein the holey fibre comprises a dopant,and wherein the dopant is selected from the group comprising Ytterbium,Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium Dysprosium,Tin, Germanium, Phosphorous, Aluminium, Boron, Antimony, Uranium, Gold,Silver, Bismuth, Lead, a transition metal, and a semiconductor.
 13. Theapparatus of claim 1, and wherein the non-linear optical devicecomprises a semiconductor optical amplifier.
 14. The apparatus of claim1, and wherein the non-linear optical device comprises one of a lithiumniobate channel waveguide, or a lithium niobate planar waveguide. 15.The apparatus of claim 1, and wherein the non-linear optical devicecomprises one of a periodically poled lithium niobate channel waveguideor a periodically poled lithium niobate planar waveguide.
 16. Theapparatus of claim 1, and wherein the non-linear optical device isselected from the group comprising an optical switch, a holey fibre, apoled-fibre, a potassium titanyl phosphate (KTP) or other crystallinewaveguide, a periodically poled KTP or other crystalline waveguide, anon-linear optical loop mirror, a Kerr gate, an optical fibre, anon-linear amplifying loop mirror, or a non-linear optical modulator.17. The apparatus of claim 1, and wherein the apparatus is furtherconfigured to modulate the first signal.
 18. The apparatus of claim 1,and wherein the apparatus is further configured to demultiplex the firstsignal.
 19. The apparatus of claim 1, and further comprising an activelymode-locked fibre laser.
 20. The apparatus of claim 1, and furthercomprising an interferometer comprising a first arm and a second arm,and wherein the first arm comprises the non-linear optical device. 21.The apparatus of claim 1, and further comprising a filter, and whereinthe filter is a wavelength selective filter.
 22. The apparatus of claim1, and further comprising a polarizing element, and wherein thepolarizing element is one of a polarizer or a polarization beamsplitter.
 23. The apparatus of claim 1, and further comprising a clockgenerator.
 24. The apparatus of claim 23, and wherein the clockgenerator is a short-pulse generator selected from the group comprisinga mode-locked fibre laser, an actively mode-locked fibre laser, agenerator comprising an electro-absorption modulator and a laser, agenerator comprising an electro-optic modulator and a laser, and again-switched laser diode.
 25. The apparatus of claim 23, and whereinthe clock generator comprises a means for pulse compression.
 26. Theapparatus of claim 25, and wherein the means for pulse compressioncomprises one of a dispersion compensator fibre, a chirped fibre Bragggrating, a dispersion decreasing fibre, an optical amplifier, a Ramanamplifier, an optical switch, or an optical pulse compressor.
 27. Theapparatus of claim 1, and further comprising a plurality of non-linearoptical devices, and wherein the apparatus is further configured todirect the second signal at the second signal input port to each of thenon-linear optical devices.
 28. The apparatus of claim 27, and whereinthe apparatus is configured as an optical multiplexer.
 29. The apparatusaccording of claim 27, and wherein the apparatus is configured as anoptical demultiplexer.
 30. The apparatus according of claim 27, andwherein the apparatus is configured as an inverse multiplexer.
 31. Theapparatus of claim 1, and further comprising a switch and a controlinput for controlling the switch.